"We may have found the hottest time the world has ever had," researcher Paul Wignall, a geologist at the University of Leeds in England, told LiveScience.

The mass extinction at the end of the Permian Era about 250 million years ago was the greatest die-off in Earth's history. The cataclysm killed as much as 95 percent of the planet's species. One key factor behind this disaster was probably catastrophic volcanic activity in what is now Siberia that spewed out as much as 2.7 million square miles (7 million square kilometers) of lava, an area nearly as large as Australia. These eruptions might have released gases that damaged Earth's protective ozone layer.

After the end-Permian mass extinction came a time "called the 'dead zone,'" Wignall said. "It's this 5-million-year period where there's no recovery, where there is a very low diversity of life."

The researchers focused on isotopes or atomic variants of oxygen within these fossils. All isotopes of oxygen have eight protons in their atomic nuclei, but differ in the number of neutrons they possess — oxygen-16 has eight neutrons, while oxygen-18 has 10.

As marine creatures form shells, bones and teeth, "they tend to use lighter isotopes of oxygen under warmer conditions," Wignall said. "You can still see this today when looking at modern-day sea creatures.

The ratios of oxygen isotopes in their shells are entirely controlled by temperature."

The researchers analyzed strange eel-like creatures known as conodonts, which are known mainly by their elaborate mouthparts. The fossils came from the Nanpanjiang Basin in south China, helping reconstruct what temperatures were like around the equator at the end-Permian.

Different groups of conodonts shed light on what temperatures were at different depths. For instance, one group, Neospathodus, lived down about 230 feet (70 meters) deep, while others, such as Pachycladina, Parachirognathus and Platyvillosus lived near the surface.

"We had to go through several tons of rock to look at tiny conodont fossils," Wignall said. "People always thought the end-Permian extinctions were related to temperature increases, but they never measured the temperature then in much detail before, since it involves a lot of hard work looking at these microfossils."

Extreme case of warming

The fruits of this labor? "We've got a case of extreme global warming, the most extreme ever seen in the last 600 million years," Wignall said. "We think the main reason for the dead zone after the end-Permian is a very hot planet, particularly in equatorial parts of the world." [The Harshest Environments on Earth]

The upper part of the ocean may have reached about 100 degrees F (38 degrees C), and sea-surface temperatures may have exceeded 104 degrees F (40 degrees C). For comparison, today's average annual sea-surface temperatures around the equator are 77 to 86 degrees F (25 to 30 degrees C).

"Photosynthesis starts to shut down at about 35 degrees C [95 degrees F], and plants often start dying at temperatures above 40 degrees C [104 degrees F]," Wignall said. "This would explain why there's not much fossil record of plants at the end-Permian— for instance, there are no peat swamps forming, no coal-forming whatsoever. This was a huge, devastating extinction."

Without plants to absorb carbon dioxide, more of this heat-trapping gas would stay in the atmosphere, driving up temperatures further. "There are other ways of taking carbon dioxide out of the atmosphere, but the planet lost a key way for millions of years," Wignall said.

These lethally hot temperatures may explain why the regions at and near the equator were nearly uninhabited. Nearly all fish and marine reptiles were driven to higher latitudes, and those creatures that remained were often smaller, making it easier for them to shed any heat from their bodies.

"I'm sure there will be questions as to whether sea-surface temperatures really did get this extreme," Wignall said. "But I think extreme temperatures would explain quite a lot with the fossils we see showing major losses of animal and plant life."

These findings show that global warming can directly cause extinctions. Still, although the world is currently warming, "we're not going to get anywhere near the level seen after the end-Permian," Wignall said. "We need to worry about global warming, but it's not going to get to this stage."

The scientists detailed their findings in the Oct. 19 issue of the journal Science.